feat: add unix socket support to deltachat-rpc-server

[Simon-Laux]

Progress

• basic implementation
• handle shutdown correctly: delete socket file again
  • we could also join tokio tasks, but it seems to work fine already without that)
• write a python client class
• write a test for it in python
• fix issues of the python class (looks like it either doesn't work at all, hangs or never exits)
• dedicated pr: different event queue for each new client
  • (but first connections/client should get all previous events)
• rebase after feat: allow to connect a python rpc-client to an rpc-server via a unix FIFO file instead of subprocess #7535 is merged

closes #7545